A passive optical network (PON) is one system for providing network access over “the last mile.” The PON is a point to multi-point network comprised of an optical line terminal (OLT) at the central office, an optical distribution network (ODN), and a plurality of optical network units (ONUs) at the customer premises. Downstream data transmissions are broadcast to all of the ONUs, while upstream data transmissions are transmitted to the OLT using time division multiple access (TDMA) or wave division multiple access (WDMA).
Many transmission systems (e.g. synchronous optical network (SONET), gigabit passive optical network (G-PON or GPON), 10G Ethernet, and others) use a scrambler to condition the transmitted bit pattern (i.e., the “line pattern”). The scrambler uses a cyclic redundancy check (CRC) shift register to generate a pseudo-random bit sequence, and this sequence is then XOR'ed with the payload data to produce the line pattern. The receiver has an identical pseudo-random bit sequence generator, and can XOR the received line data pattern with this sequence to recover the payload data. This system is very efficient; however, scrambling does have the liability that it only offers a probabilistic guarantee of good line patterns.
In general, good line patterns are those that are balanced (50% “1” and 50% “0”), and have adequate transition density. When properly operating, a scrambled sequence looks fairly random, and long runs of identical digits are unlikely, but still possible. These unlikely occurrences do indeed happen, and are one of the sources of residual bit errors. A worse problem has to do with intentional attacks by a user who wishes to disable the system. Such a user can transmit packets that have data patterns that are similar to the scrambler's bit sequence. When scrambled, such packets will result in all “0” or all “1” line patterns, and the transmission link will fail in this circumstance.